Many people assume that the number 666 is a literal mark of the beast. However a close reading of Revelation 13 shows us something quite different.
First, we must remember that we are reading an apocalyptic work - its numbers are symbolic, as is nearly everything else. 7 = perfection. 6 = imperfection. The number 6 three times (666) means Ultimate Imperfection in the apocalyptic tradition. The beast is imperfect to the ultimate degree; this would jive with John's view that Roman Imperial Power (specifically, the Emperor) was antichrist.
However, Revelation is full of double-entendres. John writes symbolically in the apocalyptic tradition, but he also means for his audience to draw a person's name from the numbers (Rev. 13.18). People have tried to match 666 with different people throughout history, including tapping Ronald Reagan as the Antichrist. However, to understand what John wanted his audience to understand, we again have to turn to the ancient traditions.
Letters in greek and hebrew are also numbers. While 666 doesn't come out to much in greek, in hebrew 666 spells the name Neron Kaisar (Nero Caesar). Drop the "n" at the end of "Neron" to spell "Nero," and you get the number 616 - which is a popular replacement for 666 in ancient manuscripts for Revelation 13.18 (check the footnote in your Bible to see). Some argue that John was writing in greek and therefore to make the switch to hebrew is ludicrous. However, John seems to have a keen interest in Hebrew words for things throughout Revelation (for example 9.11, 16.16). Also some of his readers would have come from Jewish backgrounds, so this practice would not seem strange to them. John does, after all, say that it requires wisdom to calculate the number. "Wisdom," once again reveals yet another double-entendre: (1) one who can understand the calculation of the Beast's number/name, and (2) one who can discern who is antichrist (those who oppose God).
We must remember that we have two beasts: Roman Imperial Power (esp. the Emperor Nero) and The Roman Imperial Cult. The second beast forces worship of the first. Rev. 13.16 says that is causes all to be marked with the mark of the first beast (who is represented by 666).
However, the mark is not a literal mark. Is it a seal of ownership - much like God puts a seal on his people in Rev. 7.3, 9.4. (This is also reminicent of the seal God placed on Cain after he killed Abel - a seal of ownership and, therefore, protection). The greek word used for "mark" is charagma. Charagma is the technical term used for the imperial stamp on official documents and coins. Basically, this mark showed ownership. Placing a seal on the forehead or right hand made this ownership public (as with slaves). John is not talking about a literal mark; he simply means anything that makes this ownership public. Taking the mark of the beast is not a literal bio-implant computer chip; it is anything that publically shows to whom you belong (in this case: the Beast). Worshipping the Emperor as god, therefore, obviously is "taking the mark of the beast." It shows everyone that you belong to the Emperor (as opposed to God). John also writes that one cannot buy or sell without this mark. This was true. It would hurt Christians financially if they did not accept the "mark" by participating in Emperor worship.
The seal is not a literal one, but it is a real one. By accepting a role in Emperor Worship, a person would violate the first commandment of God. We need not worry about the government instituting a mark of 666 via bio-implants, barcodes or social security numbers. We need to worry about accepting the mark of the beast John talked about: displaying ownership of our lives by someone other than God, by worshipping someone (or something) other than God.
Also in this series:
REVELATION 13: ANTICHRIST
THE BOOK OF REVELATION: PART 2
THE BOOK OF REVELATION: PART 1
THE NATURE OF PROPHECY